2022.6.24-----leetcode. five hundred and fifteen

  List<Integer> ans;
    Deque<TreeNode> q;
    public List<Integer> largestValues(TreeNode root) {
        ans=new ArrayList<>();
        q=new ArrayDeque<>();
        if(root!=null)
            q.add(root);
        bfs();
        return ans;
    }
    // By layer   Sequence traversal 
    void bfs(){
        if(q.isEmpty()) return;
        int max=Integer.MIN_VALUE;
        int n=q.size();
        for(int i=0;i<n;i++){
            TreeNode cur=q.poll();
            max=Math.max(max,cur.val);
            if(cur.left!=null)
            q.add(cur.left);
            if(cur.right!=null)
            q.add(cur.right);
        }
        ans.add(max);
        bfs();
    }